"The U.S. military’s weapons and communications systems rely on global positioning data to determine their locations. But satellite navigation systems are becoming more susceptible to jamming during a conflict.
"To meet this challenge, the Defense Department’s research and development arm has launched a program to develop new technologies that would allow military gear to function and to know its location without the benefit of Global Positioning Satellite signals. "
http://gcn.com/articles/2011/08/16/darpa-develops-satellite-free-navigation-systems.aspx
